The purpose of this chapter is to understand how human exposure to short-term (air pollution episodes) or long-term pollution can cause adverse acute or chronic health effects. This chapter provides knowledge of the health effects including understanding of the nature of health risks associated with air pollution episodes as compared to everyday exposures; becoming familiar with types of scientific evidence used to determine potential cause–effect relationships; recognizing the importance of epidemiological studies in establishing a cause–effect relationship and inherent limitations in conducting epidemiological studies of community-wide air pollution exposures; awareness of the nature of toxicological studies and their role in establishing cause–effect relationships; recognizing that air pollutant exposures affect a limited number of target organs in the human body; awareness of respiratory system anatomy and defense mechanisms; and understanding of the potential role of pollutant exposures in causing or aggravating chronic bronchitis, pulmonary emphysema, lung cancer, bronchial asthma, and respiratory infections. Common air pollutants such as CO, O3, SO2/sulfates, particulate matter, NOx, and Pb have been identified by scientists with their significant linkage to specific diseases and have been regulated under air quality standards. The basic principles of risk assessment and risk management are the protection of the general population from ambient air pollution and reduction of their exposure, although the related diseases could result from other personal behaviors or habits like tobacco smoking.